Apparel: Clothing (Source)
Can I say "I want to buy many apparels" or "I want to buy much apparel"?
"Apparel" is not countable. You cannot say, "I bought three apparels yesterday." You could technically say "much apparel", but "much" is rarely used this way; I think it's rather out of date. Actually "apparel" is a rather rarely-used word today. English speakers these days tend to say "clothing" or "clothes". A fluent speaker would probably say, "I want to buy a lot of clothes". "A lot" is informal, so if you wanted to be more formal you'd say, "I want to buy many items of clothing" or "... many articles of clothing."
"Apparel" is, I think, generally used only to describe a type of clothing. Like, "The guests at the ball all wore formal apparel" or "The dancers were dressed in traditional Romanian apparel". It would not be wrong to say, "The apparel in my closet includes three blue shirts and two red shirts", but few modern English speakers would say that.